There is a growing interest in antistatic materials and coatings in various fields of technology, particularly in the photographic and electronics industries. Antistatic materials (i.e., antistats) are electrically conductive materials. They utilize a conduction process that results in the dissipation of electrical charges (i.e., "static electricity"). Thus, it is desirable to use antistats in applications in which it is necessary to avoid the build-up of electrical charges, which can discharge suddenly and produce a detrimental effect. For example, in photographic applications, an antistatic coating on film avoids sudden discharges of built-up electrical charge that can cause undesirable recordation of the associated flash of light.
Preferred antistats are those that conduct electrons by a quantum mechanical mechanism rather than by an ionic mechanism. This is because antistats that conduct electrons by a quantum mechanical mechanism are effective independent of humidity. That is, they are suitable for use under conditions of low relative humidity, without losing effectiveness, and under conditions of high relative humidity, without becoming sticky. A major problem, however, with such electron-conducting antistats is that they generally cannot be provided as thin, transparent, relatively colorless coatings by solution coating methods. Although there have been many attempts to do so, such as by using defect semiconductor oxide particle dispersions and conductive polymers, there has been very little success in overcoming this problem. The use of vanadium oxide has proven to be the one exception, however. That is, effective antistatic coatings of vanadium oxide can be deposited in transparent, substantially colorless thin films by coating from aqueous dispersions.
Three unique properties that distinguish vanadium oxide from other antistatic materials are its conduction mechanism, dispersibility, and morphology. The latter two properties are generally highly dependent upon the method of synthesis, the first somewhat less so. The conduction mechanism in vanadium oxide is primarily a quantum mechanical mechanism known as small polaron hopping. By this mechanism, electrons are transported through the material by transmission (i.e., by "hopping") from one vanadium ion to the next. This conduction mechanism does not require the presence of a well-developed crystalline lattice or a specific defect structure, as do defect semiconductors such as doped tin oxide or doped indium oxide.
Because small polaron hopping electronic conduction does not require a well-developed crystalline structure, there is no need for an annealing step when a film or coating is made from vanadium oxide. Furthermore, vanadium oxide is conductive simply upon precipitation or formation in solution, without being adversely affected by changes in relative humidity. Thus, a highly dispersed form of vanadium oxide that exhibits electronic conductivity, and desirable morphology, particle size, and dispersion properties is useful for the preparation of conductive antistatic coatings.
The effectiveness of a dispersed form of vanadium oxide, i.e., a vanadium oxide colloidal dispersion, for the preparation of antistatic coatings can be expressed in terms of the surface concentration of vanadium. The surface concentration is described as the mass of vanadium per unit surface area, i.e., mg of vanadium per m.sup.2 of substrate surface area, required to provide useful electrostatic charge decay rates. Generally, the lower the surface concentration of vanadium needed for effective conductivity in an antistatic coating, the more desirable the vanadium oxide colloidal dispersion. This is because with a lower surface concentration of vanadium, there is typically less color imparted to the coating, the coating is more transparent and uniform, and in some circumstances the coating generally adheres better to the substrate and may even provide better adhesion for subsequent layers.
In the mid-1970s, Claude Guestaux of Eastman Kodak reported that a previously known synthetic method gives a vanadium oxide colloidal dispersion which, at the time, was considered uniquely useful for the preparation of antistatic coatings. Guestaux's method was based on a process originally described by E. Muller in 1911 in Z. Chem. Ind. Kolloide, 8, 1911, p. 302. The method is described in U.S. Pat. No. 4,203,769 (Guestaux) and consists of pouring molten vanadium pentoxide into water. The process has several drawbacks, however. These drawbacks include high energy requirements, the need for special reactor materials and equipment, and the creation of conditions which generate toxic vanadium oxide fumes. Furthermore, the Guestaux method results in incomplete dispersion of vanadium oxide. The nondispersed vanadium oxide must then be removed from the viscous dispersion; however, such viscous vanadium oxide dispersions are usually very difficult to filter.
There are several other methods known for the preparation of vanadium oxide colloidal dispersions. These include inorganic methods such as ion exchange acidification of NaVO.sub.3, thermohydrolysis of VOCl.sub.3, and reaction of V.sub.2 O.sub.5 with H.sub.2 O.sub.2. However, vanadium oxide colloidal dispersions prepared by these methods using inorganic precursors are much less effective for the preparation of antistatic coatings than colloidal dispersions prepared by the process described by Guestaux in U.S. Pat. No. 4,203,769. To provide coatings with effective antistatic properties from dispersions prepared from inorganic precursors typically requires substantial surface concentrations of vanadium. These surface concentrations of vanadium generally result in the loss of desirable properties such as transparency, adhesion, and uniformity.